THE RELATIONSHIP BETWEEN TRANSCRIPT EXPRESSION LEVELS OF NUCLEAR ENCODED (TFAM, NRF1) AND MITOCHONDRIAL ENCODED (MT-CO1) GENES IN SINGLE HUMAN OOCYTES DURING OOCYTE MATURATION
Ghaffari Novin M, Allahveisi A, Noruzinia M, Farhadifar F, Yousefian E, Dehghani Fard A, Salimi M
*Corresponding Author: Dr. Azra Allahveisi, Department of Anatomical Sciences, Faculty of Medicine, Kurdistan University of Medical Sciences, Pasdaran Street, Sanandaj, Iran. Tel: +98-873-664-673. Fax: +98-873-364- 674. E-mail: allavaisie@gmail.com
page: 39

INTRODUCTION

Oocyte maturation and oocyte quality are important parameters in successful reproductive results of the assisted reproduction technologies (ART). Mitochondria are the most abundant organelles in the cytoplasmic oocyte [1]. They are necessary for adenosine triphosphate (ATP) generation through oxidativephosphorylation (OXPHOS), unlike any other process of the cell, and depend highly on the expression of proteins encoded by the mitochondrial and nuclear encoded gene [1,2]. Mitochondria are inherited exclusively from the mother [3]. Indeed, mitochondrial function is associated with mitochondrial DNA (mtDNA) [4,5]. The mitochondrial genome encodes essential proteins, which are crucial for the generation of ATP. These proteins are transcribed and translated in the mitochondrial matrix [6]. Human oocyte mitochondrion has only a single copy of the genome, which is representative of the mitochondrial number. However, it has been clearly documented that mtDNA number is expanded during oocyte growth [7]. Mitochondrial biogenesis is managed independently from the nuclear genome and depend on regulatory coordination between the nuclear and mitochondrial genomes [8]. However, little is known about the regulation of mitochondrial gene expression, compared with nuclear genes [8]. Regulatory coordination of the nuclear and mitochondrial genes are important in cell survival and energy homeostasis [9]. Two products of these genes, NRF1and TFAM, are well-known essential ubiquitous factors for the mtDNA replication and expression [10]. The NRF1 gene contributes in regulating the expression of nuclear encoded components of the mitochondrial respiratory chain [11]. The human TFAM is a 25 kD protein with nuclear-encoded high-mobility group (HMG) box protein, which plays an important role through sequence binding to the heavy strand promoter (HSP) and light strand promoter (LSP) sites in the D-loop of human mtDNA, a control region that regulates mtDNA transcription and replication [12- 14]. The TFAM gene has various functions, including packaging mtDNA into a nucleoid-like complex and maintaining and repairing mtDNA molecules [15,16]. NRF1 has also been shown to be linked to the promoters of the TFAM gene [17]. Their important roles have been demonstrated by transgenic experiments, showing that depletion of mtDNA in TFAM and NRF1 homozygous knockout mice resulted in the animals’ deaths [18,19]. Another factor, MT-CO1, is a terminal component and one of the three genes of mitochondrial respiratory chain, encoded by the mtDNA [19-21]. Transcription levels of the MT-CO1 gene may possibly be an indirect indicator of the mtDNA metabolic activity. Active transcription of the mitochondrial genome has been demonstrated to initiate at various developmental stages, depending on the species [22]. Reports from many studies suggested that gene-specific transcription factors directly affect gene transcription in mitochondria [23]. However, relationship between the expression levels of nuclear and mitochondrial encoded genes during human oocyte maturation is not well understood. The aim of the current study was to quantify the relationship between relative expression levels of NRF1 and TFAM, and the MT-CO1 genes in single human oocytes at various stages of the human oocyte maturation from germinal vesicle (GV) stage to metaphase II (MII) stage.



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